Electronic Design

Testing Of MPEG-2 Set-Top Boxes Must Be Fast, Thorough

In approximately 10 seconds, individual tests must verify that a fully functional set-top box will be delivered to the consumer.

In a relatively short time, MPEG encoding technology has completely changed the landscape of television content delivery to consumers. Whether through satellite, cable, or over-the-air, the efficiencies of MPEG-2 compression have enabled a veritable revolution within these industries, leading to set-top box development.

These set-top box designs go through a rigorous process to verify that individual parts of the system, such as RF receivers, demodulators, MPEG-2 processors, video and audio outputs, and analog RF modulators, work as one unit. However, testing during the mass manufacturing of set-top boxes is relegated to approximately 10 seconds per required test.

Within this short time span, a complex system goes through testing and evaluation to deliver a working unit to consumers. With production volumes of up to 10,000 units per day, the individual tests designed to be performed in the allotted 10 or so seconds must verify the integrity of the set-top box.

While MPEG-2 compression technology has changed the content delivery landscape to the consumer, the viewing experience is technologically the same analog experience as it has been since the dawn of the television era. Today's television sets are analog devices that require traditional signal sources derived within the set-top box from the MPEG-2 transport stream. Computer monitors and professional video monitors have digital video, and possibly digital audio, inputs. But, analog consumer television sets will dominate the living room for the foreseeable future.

Although this article focuses on MPEG-2 decoder problems that can be detected during manufacturing, remember that the set-top box system is symbiotic. Both the digital and analog sections rely on each other to produce acceptable pictures and sound. Because the system is interrelated, recognizing whether a problem is due to an error in the MPEG-2 system or in the analog system, and the correction of such errors, is the goal of the production line test.

Testing the system: An MPEG-2 set-top box will often be segmented into three separate sections or boards. Figure 1 shows a detailed example of the inputs, outputs, and each component within the set-top box. After assembly of the set-top box chassis (without its cover), several inputs and outputs are connected to verify its operation (Fig. 2).

Production test begins with a power-up test, which is as simple as it sounds. The power button is engaged and the unit comes to life. It typically has LED indicators on the front panel, and a manufacturer's logo, stored in the set-top box's memory, displayed on a monitor or television screen. While this might be the first indication of possible problems with the video section (if the LEDs light up but the logo doesn't appear or has chroma problems), this test is just for power-up. Regardless of what does or doesn't appear on the screen, if the unit powers up, it passes the test.

Once the set-top box is powered up, several independent tests are performed, taking about 10 seconds per test. The set-top box will be connected to various sources and analyzers to conduct and verify each test (Fig. 2). These sources include:

  • At least two analog programs, audio and video, modulated to over-the-air channels (typically channels 2 and 13, as they're at opposite ends of the VHF band);
  • At least two multiprogram digital MPEG-2 transport streams (with one program set for conditional access), modulated and upconverted to UHF L-band frequencies.

Both the analog and digital RF signals are combined to feed the RF input of the set-top box. The analog outputs of the set-top box are connected to separate video and audio analyzers, as well as to a video monitor and speakers (via an audio amplifier). If the set-top box contains a channel 3-4 modulator, then a frequency analyzer and a television set will also be connected to it.

Although the analyzers look at the technical characteristics of the audio and video signal, the manufacturing-line technician must watch the monitor and listen to the speakers. A variety of errors can "look" to an analyzer as though no error exists. For example, macroblocking (discussed below) technically conforms to all analog video requirements, but it provides an unacceptable image to the viewer.

Analog system tests: Analog tests verify that the analog tuner, demodulator, and outputs of the set-top box work correctly. Also checked in this series of tests is the remote control, used to select channels.

The analog test series includes:

  • Select the first analog channel using the remote control (remote-control test);
  • Verify the presence of the analog signal (test pattern) using the video analyzer (analog-tuner, demodulator, and video-output test);
  • Make appropriate video-level adjustments to the NTSC/PAL and S-Video outputs using the test pattern and video analyzer (video-output test);
  • Verify the stereo audio frequency and amplitude using an audio analyzer (analog-tuner, demodulator, and audio-output test);
  • Select the second analog channel and quickly verify that the tuner locks to the second analog test pattern and stereo signal (analog-tuner test).

Digital system tests: Evaluating the digital system is more complex than the analog system test because the digital section relies on the analog section for audio and video output. This is why the analog system tests are completed first. Upon their completion, the following tests are performed:

  • Select the first program (test pattern) from within the first multiprogram digital carrier (digital-tuner and demodulator test);
  • Verify that the NTSC/PAL and S-Video signals are within tolerance using a test pattern and video analyzer—making adjustments on the analog board if necessary (MPEG-2 demultiplexer, MPEG-2 video-decoder, and video-output test);
  • Verify the stereo audio frequency and amplitude using an audio analyzer (MPEG-2 audio-decoder and audio-output test);
  • Select the first program (conditional access) within the second digital carrier (digital-tuner and demodulator test);
  • Using a SmartCard, verify that the set-top box allows access to the scrambled program (SmartCard interface and MPEG-2 decoder test);
  • Using a color-television picture monitor and stereo speakers, subjectively verify that the moving video sequence flows without any glitches, macroblocks, or lip-sync errors on the audio (MPEG-2 demultiplexer and decoder test).

There are optional tests if either HDTV or Dolby Digital is supported:

  • Select the second program from the first digital carrier;
  • Verify and adjust analog high-definition levels for RGB and Y/Pb/Pr (color-difference) outputs for both 720-progressive and 1080-interlaced scans (using test pattern);
  • Verify stereo audio frequency and amplitudes, as well as the serial output from the Dolby Digital port;
  • Lastly, select the second program from the second digital carrier and subjectively check for visual errors using a high-definition monitor, or a VGA monitor if RGB outputs are provided.

Errors can exist in the MPEG-2 decoder section and in the analog section. Luckily, MPEG-2 errors differ from analog errors in appearance, aiding in diagnosis. Below are common MPEG-2 and analog errors with their possible causes:

Jittery video: While difficult to show in print because of their temporary basis, jitter errors occur frequently in real life with digital television systems. Problems with the program-clock-reference (PCR) values can lead to chroma and jitter problems. The color-burst carrier delivered via the NTSC/PAL signal is now delivered at around 25 times per second as a 42-bit counter. Figure 3 shows the PCR value with the 188-byte packet. Problems with originating these accurate time stamps sometimes happen, but there can still be occasions where they're delivered accurately, while the set-top box fails to accurately convert them back into a stable 27-MHz timing reference, required for the chroma signal. In the manufacturing environment, the source signal should be accurate, so all that's necessary is verification that the chroma is valid (based upon a reference test pattern), and the moving video sequence plays smoothly.

Macroblock video: There are numerous ways by which macroblock errors can be introduced into the system (Fig. 4). The most common is if the RF input fails to receive enough power. There may be enough to decode some pictures, while others could be impaired from the introduction of corrupted bits. Digital television has a "cliff effect," which allows pictures to look very clear sometimes, while macroblocks and freeze-frames appear at other times. Given sufficient RF power and no faults with the internal components, the video should always look very clean.

Chroma: Because most digital set-top boxes drive analog outputs, there's room to introduce luma and chroma errors. If the analog outputs aren't calibrated using a reference pattern and a video analyzer, there might be a problem with the luma and chroma when viewed on a color-picture monitor.

Conditional access (Pay Per View): Conditional access to scrambled programming is available through a SmartCard interface on the set-top box. It's triggered by the "transport scrambling control" 2-bit parameter, which is the sixth parameter in Figure 3. One way to test this feature is to make one of the moving programs on the digital carrier a scrambled program. Tuning to this program and viewing a clean moving picture will verify that the SmartCard interface and the supporting circuitry work well. If the program isn't viewable, the error is typically in the SmartCard interface. Usually there's no time to test for unauthorized conditional access, where unauthorized users can receive a program.

Audio and video synchronization (lip-sync): If it's known that the audio and video in the transport stream are synchronized (a Tektronix VM700T provides an objective lip-sync measurement to quantify the delay), and there's an obvious lip-sync error from the set-top box output, the culprit is the MPEG-2 decoding section. This error can occur at random times. The simplest solution is to power down the unit and restart the test. This allows the set-top box to re-initialize. If the error continues, then the set-top box is faulty.

No video or no audio: This problem happens frequently in the design phase, but hopefully never within the production process. Assuming the analog output section is working properly, the cause is usually from the originating MPEG-2 multiplexer. It also occurs when the decoder fails to extract the video and audio elementary stream program identifier (PID) values. If the set-top box fails to find the program association table (PAT) and program map tables (PMTs), also known as the electronic program guide, the video or audio will usually be absent. Sometimes, missing video will cause the decoder to just repeat the frame from the last channel to which it was tuned. If the set-top box finds the tables and identifies the correct PID values, but a problem now exists within the demultiplexer or decode section, the video or audio will still be absent (see the demultiplexer and descrambler board in Fig. 2).

A number of factors cause these errors:

Solder bridges across MPEG decoder chips: Bridges and opens commonly cause circuit-board failures within production. All the above symptoms can be seen depending on which component is bridged or left open. This type of problem means the entire set-top box will be rejected and further tested and evaluated. High occurrences of this type of failure would hopefully result in a revisit to the solder and ECB process to eliminate as many failures as possible.

Firmware errors:
Problems with software, firmware, and hardware definitely happen in the design phase, but hopefully never on the production line. After manufacturing engineering has thoroughly tested the pilot runs of the first set-top boxes, the only types of software errors allowed are those derived from failures of EPROM, DRAM, and flash memory. Because the mean time between failures (MTBF) is never infinity, hopefully CRCs and checksums will catch most problems. If not, the fact that the production procedure needs changing carriers and programs, along with verification of test and moving patterns, will help catch any memory problems.

Bad decoder chips: Normally, bad decoder chips are weeded out during the design phase, and sometimes during the evaluation of the initial pilot builds of the decoders. If a problem occurs on the production line, it might happen when the original decoder part goes obsolete and is replaced by a nearly identical part. Once again, if the new part isn't perfect, it should be caught by manufacturing engineering, not by the production technician.

Given the goal of manufacturing is to produce huge quantities of set-top boxes in the shortest period of time, it's important to focus on the inputs and outputs, and just on a few internal tests. It's even better if the I/O test incorporates the testing of the embedded circuits. High-volume production with one minute per box, or 10 seconds per test, means that a lot of engineering testing must take place before starting production.

In-depth RF and MPEG testing is required by engineers to verify that all of the real-world combinations can be met. Obviously, different power and noise levels will occur, as well as a wide range of video bit rates from 2 to 18 Mbits/s. After verifying all in-depth tests, production can choose a small subset of tests to ensure that the set-top box will be usable by the consumer under a wide variety of conditions.

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